DIVERSITY OF CLADOCERA ( CRUSTACEA : BRANCHIOPODA ) IN FLOODPLAIN LAKES OF MANIPUR , NORTHEASTERN INDIA

Cladocera, a group of the branchiopod crustaceans, serve as important fish-food organisms, comprise an integral component of metazooplankton and contribute significantly to secondary productivity in lentic freshwater biotopes. These entomostracous Crustacea have been studied from distant parts of this country since about one and half century but the Indian literature shows paucity of works on their ecosystem diversity in aquatic environments in general and in the floodplain lakes of India in particular (Sharma and Sharma, 2008). Referring to the later, important contribution is limited to the recent study by Sharma and Sharma (loc cit.) in the floodplain lakes (beels) of the Brahmaputra river basin, Assam, N.E. India.


INTRODUCTION
Cladocera, a group of the branchiopod crustaceans, serve as important fish-food organisms, comprise an integral component of metazooplankton and contribute significantly to secondary productivity in lentic freshwater biotopes.These entomostracous Crustacea have been studied from distant parts of this country since about one and half century but the Indian literature shows paucity of works on their ecosystem diversity in aquatic environments in general and in the floodplain lakes of India in particular (Sharma and Sharma, 2008).Referring to the later, important contribution is limited to the recent study by Sharma and Sharma (loc cit.)  individual species were isolated and were identified following the works of Srnimov (1971Srnimov ( , 1976Srnimov ( , 1992Srnimov ( , 1996)), Michael and Sharma (1988), Sharma and Sharma (1999) and Orlova-Bienkowskaja (2001).Quantitative plankton samples were analyzed for enumeration of the cladoceran densities (n/1).
Percentage similarities between cladoceran communities were calculated vide Sorensen's index and were analyzed by hierarchical cluster analysis (SPSS version 10).Species diversity (Shannon's index), dominance (Berger-Parker's index) and evenness (Pileou's index) were calculated following Ludwing and Reynolds (1988) and Magurran (1988).The significance of temporal variations of richness and densities were ascertained by ANOVA.Ecological relationships were analyzed with simple correleation coefficients (r) and multiple regression (R 2).

Cladocera composition and distribution
Cladocera, second important qualitative component of zooplankton of all the pats of Manipur, reveal in all 56 species and comprise a significant fraction (50.9%) of the Indian Cladocera.The qualitative diversity assumes importance in light of a conservative estimate (Sharma and Michael, 1987;Sharma, 1991) of occurrence of up to 60-65 species of these branchiopod Crustaceans from tropical and subtropical parts of India.Amongst 42 genera and 10 families of freshwater cladocerans so far known from India (BKS, unpublished), 30 genera and 7 families are represented in our collections.Number of the examined species and higher taxa reflect rich and diverse taxocoenosis of Cladocera.The recorded families represent two phylogenetic stems of this group (Dumont and Negrea, 2002) namely the Ctenopoda and the Anomopoda; the former includes only the family Sididae while six families of the latter (Daphniidae-Bosminidae-Moinidae-Macrothricidae-II yocryptidae-Chydoridae) are noticed in our collections.
Overall Cladocera richness observed in the pats of Manipur tallies with their report from the floodplain lakes (beels) of the Brahmaputra river basin of Assam (Sharma and Sharma, 2008).On the other hand, the richness is notably higher than the reports of 36 species from 20 floodplain wetlands of Southeastern West Bengal (Khan, 2003), 11 species from two floodplain lakes (Khan, 1987) of Kashmir, 9 species from 65 wetlands of 24-Parganas district (Nandi et al., 1993) of West Bengal, 12 species (Sanjer and Sharma 1995) from floodplains of Bihar and 14 species from 37 floodplain lakes (Sarma, 2000) of Assam.Total richness is even higher than 41 species reported in state faunas of Meghalaya (Sharma and Sharma, 1999) and Tripura (Venkataraman and Das, 2000) from Northeastern India.site.The present report differs prominently than only Chydoridae, the largest family of Cladocera, is highly 12 species (including certain un-determined and speciose (31 species, 55.3%) and also reflects rich higher doubtful species) listed earlier by Shyamananda Singh diversity (16 genera).This family, in turn, includes members of two sub-families namely Aloninae (13 species) and Chydorinae (18).The greater richness of the chydorids confirms with the composition of the Indian Cladocera (Sharma, 1991) in general and with the floodplain lakes of Assam (Sharma and Sharma, 2008) in particular.The sub-dominant families, Daphniidae (9 species) > Macrothricidae (6 species) > Sididae (5 species), together comprise an important component (35.7%).Among 30 genera, only Alona (10 species) > Simocephalus (6 species) are more speciose.
Interestingly, lack of any species of Daphnia (the globally highly speciose genus of Cladocera) in the pats of Manipur is noteworthy.This interesting feature assumes special importance in light of earlier remarks (Sharma, 1991) on very limited distribution of this member of the Daphniidae in aquatic ecosystems of Northeastern India.
Amongst biogeographically interesting species (Sharma and Sharma, 2008)  The Cladocera are invariably considered as a group showing cosmopolitan distribution while recent biogeographical considerations comment on careful analysis of cosmopolitan nature of various species and of geographical vicariants (Dumont and Negrea, 2002).

The present study indicates dominance of cosmopolitan
Rec. zool.Surv.India species (42.8%) while cosmotropical and circumtropical species, together, form an important component (35.7%).These features impart a general 'tropical character' to the studied fauna and concur with the composition of other tropical Cladoceran faunas (Fernando, 1980;Fernando and Kanduru, 1984;Dussart et aI., 1984;Sharma, 1991).The cladoceran communities are characterized by dominance of the littoralperiphytonic taxa particularly of the Chydoridae, Macrothricidae, Ilyocryptidae and littoral species of Pseudosida, Sida and Simocephalus.On the other hand, fewer planktonic species include Bosmina longirostris, Bosminopsis deitersi, Ceriodaphania cornuta, Diaphanosoma sarsi, D. excisum, Moina micrura and Moinodaphnia macleayi and even these exhibit restricted occurrence in different pats.Further, only 12 species (21.4%) occur in all the lakes and may be categorized as common while a majority of the rest (25 species, 46.4%) are rare and occur in fewer pats.

Cladocera richness and community similarities
The present study indicates notable variations in Cladoceran richness (Table -2) in the different pats (21-51, 27 ± 7 species); the recorded range is higher than the results in the beels (21-39 species) of Assam (Sharma and Sharma, 2008).Distinctly higher richness noticed in Loktak Pat reflects micro-habitat diversity resulting from diverse nature of aquatic macrophytes in this pat and, hence, affirms greater environmental heterogeneity of the Ramsar site than the remaining pats.This statement is also affirmed by a relatively narrow range (21-31 species) of richness in the other sampled pats.Family Chydoridae (31 species) distinctly dominates cladoceran richness in all individual pats (9-29,17 ± 9 species); peak chydorid richness is observed in Loktak Pat and the number of species in rest of the Pats varies between 9-18 (12 ± 2 species).
The cladoceran communities of the sampled pats depict 52.3-84.7%similarities (vide Sorenson's index); peak similarity is noticed between Waithou Pat and Utra Pat and lowest value is recorded between Loktak Pat and Ngagua Pat (Table-3).Only few instances in the matrix indicates similarity> 60% or < 80.0% and it ranges between 60-70% and 70-80% in 47.6% and 43.8% instances respectively.Relatively higher similarities result from common occurrence of several cosmopolitan Dadaya macrops (Daday, 1898) A. quadrangularis (O.P. Miiller, 1776) A. rectangula Sars, 1862 Euryalona orientalis (Daday, 1898) Camptocercus uncinatus Smirnov, 1973 Karualona karua (King, 1853)   On the other hand, the cladoceran communities of Loktak Pat differ notably in their composition from rest of the sampled pats; this feature may be ascribed to distinctly higher richness in this Ramsar site together with occurrence of seven rare species observed only this lake as well as presence of eight other rare species with limited distribution in other pats.In general the recorded ranges correspond with their counterparts from Assam (Sharma and Sharma 2008).
The diversity registers significant temporal differences between seasons (F3, 59 = 21.128,P < 0.005 and between pats (F 14 ,59 = 5.017, P < 0.005).The notable feature of high species diversity with relatively lower densities of a large number of species observed in this study may be ascribed to fine niche portioning amongst cladoceran species in combination with high micro-and macroscale habitat heterogeneity, especially in the littoral environments as hypothesized by Segers (2008).
Further, the present results exhibit lower Cladocera dominance (0.041 ± 0.011 -0.104 ± 0.020) signifying quantitative influence of fewer species; ANOVA registers its insignificant variations between seasons as well as the different pats.The stated feature is reaffirmed by higher cladoceran evenness in various pats (0.880 ± 0.033 -0.986 ± 0.020) indicating an equitable abundance of different species.ANOVA indicates significant variations of evenness in the different pats (F 14,59 = 3.835, P < 0.005) while it shows insignificant seasonal variations.Evenness is inversely correlated with dominance (r = -0.904)while it is positively correlated with density (r = 0.498).The salient features of lower dominance and higher evenness concur with earlier remarks of Sharma and Sharma (2008).

SUMMARY
in the floodplain lakes (beels) of the Brahmaputra river basin, Assam, N.E.India.The present study on the cladoceran diversity of fifteen floodplain lakes (pats) of Manipur, therefore, merits special mention for its biodiversity and ecological importance.The observations are made on faunal diversity, nature and composition, distribution of important species and community similarities of Cladocera in the different pats.In addition, the study deals with seasonal variations in richness and abundance, and their species diversity, dominance and evenness in the sampled lakes.Individual and cumulative influences of the abiotic factors on richness and abundance are analyzed and comments are made on ecology of Cladocera.MATERIALS AND METHODS The observations were undertaken (during November 2002-0ctober 2003) in fifteen floodplain lakes (pats) of the Iral, Imphal and Thoubal river basins (Longitude: 93° 45'-94° 00' E, Latitude: 24° 25'-24° 45' N) and located in Bishnupur, Imphal and Thoubal districts of Manipur.plants noticed at the sampled sites of these pats include Eichhornia crassipes, Hydrilla verticellata, Euryale ferox, Vallisnaria spira lis, Utricularia flexuosa, Trapa natans, Lemna trisuIa, Pistia striates, Salvinia sp.Nymphaea spp., Nymphoides spp., Nelumbo mucifera, Potamageton spp.and Azolla pinnata.It is not possible to categorize different pats because of heterogeneous occurrence of different aquatic macrophytes but all the stated species occur in Loktak Pat.Water samples were collected seasonally from different pats during study period and were analyzed for water temperature, specific conductivity, pH, dissolved oxygen, alkalinity and hardness.Besides, qualitative and quantitative plankton samples were collected seasonally, from 4-5 sites each based on habitat variations, from various pats with nylobolt plankton net (Mesh size: 50 ]lm); the former were obtained by towing and the later by filtering 25 litres of water each.All the plankton samples were preserved in 5% formalin.Qualitative samples were screened; Pat, 2-Waithou Pat, 3-Utra Pat, 4-Sana Pat, 5-Lakoi Pat, 6-Takmu Pat, 7-Ikop Pat, 8-Kharung Pat, 9-Khoidum Pat, 10-Lousi Pat, ll-Karam Pat, 12-Ngagua Pat, 13-Tankha Pat, 14-Lamphel Pat, 15-Pumlen Pat

Fig. 1 :
Fig. 1 : Dendrogram showing Hierarchical cluster analysis of Cladocera communities of different lakes (pats) of Manipur Plankton samples collected from fifteen floodplain lakes (pats) of Manipur indicate 56 species of Cladocera belonging to 30 genera and 7 families and exhibit rich and speciose composition.The cladoceran fauna shows dominance of the Chydoridae, cosmopolitan > cosmotropical species and general tropical character.Cladocera comprise the second important qualitative and quantitative group of zooplankton, following Rotifera, in all the pats.Richness ranges between 21-51 species and records 52.3-84.7%community similarities (vide Sorenson index).Cladocera form between 24.5 ± 3.4 -31.7 ± 1.9% of zooplankton abundance, follow indistinct quantitative seasonal patterns but show maxima and minima in majority of pats during winter and monsoon respectively.The cladoceran communities are characterized by moderately high species diversity, high evenness and low dominance in all the pats.ANOVA depicts significant variations of richness and density between pats as well as seasons.Both richness and density are inversely correlated with alkalinity and positively with hardness while richness is also inversely correlated with dissolved oxygen.Multiple regression registers moderate cumulative influence of six abiotic factors on richness and abundance.

Table - I
: Abiotic factors of Floodplain Lakes (Pats) of Manipur as well as environmental heterogeneity of this Ramsar important beel of N.E.India.
It shows notable differences (12-42 species) in different seasons in individual pats (Table-4) with peaks during winter and autumn in 13 and 2 pats respectively while lowest richness is noted during monsoon in all the pats.The present results differ from indistinct seasonal maxima reported in the floodplain lakes of Assam(Sharma and   Sharma, 2008)while correspond with the latter in